In Xcode 4: Select your .xcdatamodel -> Editor -> Add Model Version.
In Xcode 3: Design -> Data Model -> Add Model Version.
You will see that a new .xcdatamodel is created in your .xcdatamodeld folder (which is also created if you have none).

2. Save.

3. Select your new .xcdatamodel and make the change you wish to employ in accordance with the Lightweight Migration documentation.

4. Save.

5. Set the current/active schema to the newly created schema.

With the .xcdatamodeld folder selected:
In Xcode 4: Utilities sidebar -> File Inspector -> Versioned Core Data Model -> Select the new schema.
In Xcode 3: Design > Data Model > Set Current Version.
The green tick on the .xcdatamodel icon will move to the new schema.

6. Save.

7. Implement the necessary code to perform migration at runtime.

Where your NSPersistentStoreCoordinator is created (usually AppDelegate class), for the options parameter, replace nil with the following code: